fix DOWNGRADED bit unpreserved

Co-authored-by: Jonas Böttiger <jonasboettiger@icloud.com>

library/std/src/sync/rwlock/tests.rs

@@ -558,7 +558,7 @@ fn test_downgrade_atomic() {
     // modify the protected value.
 
     // `W` is the number of evil writer threads.
-    const W: usize = if cfg!(target_pointer_width = "64") { 100 } else { 20 };
+    const W: usize = 20;
     let rwlock = Arc::new(RwLock::new(0));
 
     // Spawns many evil writer threads that will try and write to the locked value before the

library/std/src/sys/sync/rwlock/queue.rs

@@ -395,9 +395,9 @@ impl RwLock {
             node.next.0 = AtomicPtr::new(state.mask(NODE_MASK).cast());
             node.prev = AtomicLink::new(None);
 
-            // Set the `QUEUED` bit and maintain the `LOCKED` bit.
+            // Set the `QUEUED` bit and preserve the `LOCKED` and `DOWNGRADED` bit.
             let mut next = ptr::from_ref(&node)
-                .map_addr(|addr| addr | QUEUED | (state.addr() & LOCKED))
+                .map_addr(|addr| addr | QUEUED | (state.addr() & (DOWNGRADED | LOCKED)))
                 as State;
 
             let mut is_queue_locked = false;